Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 4: Journal app and file I/O
Lecture: Text-based File I/O and with

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0:00 So it's time to add persistence across running sessions
0:03 by saving and loading our journal from files.
0:07 Let's start by first saving our journal to a file,
0:11 so I want to come down here and any time that you work with file I/O,
0:15 you always start with this built in called open.
0:18 So here we can pass the file name by default you see the mode is R
0:22 that's read only text but we are going to change that to W for write,
0:26 and we are not really going to worry about the other options,
0:29 so we need a file name that goes here and we want to have
0:33 the mode to be write, because we are going to create or replace a file,
0:36 we are not going to just read one,
0:38 and we'll catch this and I call it fout, from file output stream,
0:43 you can call it whatever you like, it's just a variable.
0:45 The first thing we really have to do to make this work
0:47 is actually get this file name here, so I could say,
0:51 let's actually have a little folder,
0:52 that we are going to save our files into a directory,
0:56 so let's call this journals, let's suppose we want to put them in there,
1:00 and I could say something like this,
1:02 I could say file name equals, journals/
1:06 and then there is this name
1:08 remember we are passing in this journal name here,
1:10 so let's say name, and let's suppose on the end
1:12 when I have a jrl file extension to indicate our custom journal format or whatever,
1:18 it's hard it's going to be worthy of that but here we go.
1:20 Now technically, this would work on OS X and Linux,
1:23 it may not work on Windows, right, remember,
1:27 Windows uses backslashes to separate directories, not for slashes,
1:31 but more important than that, if I am combining multiple directories,
1:36 say if I am giving a like a base directory equals something like this
1:41 until this slash, and I am giving a relative directory = data/temp.
1:50 If I am given these two and I want to build up the full path,
1:53 I would say full file = base_dir + and I need to use a little separator here
1:59 because you can see on this part there is no separator up there
2:03 and then we'll say + relative directory, like so
2:06 and if this is passed me well do I know whether there is this little slash on the end
2:12 do I need to put it here, do I check, all those little nuance details
2:16 in Python we don't actually need to deal with any of that
2:19 and we get OS independent path operations,
2:23 all I am using is this module called OS
2:26 so I want to come up here, I want to import OS, and down here,
2:30 I can say instead of doing this thing,
2:32 I would like this file to be os.path. and we can just say join,
2:37 and put a whole bunch of little paths to segments together,
2:40 so we can say ./journals, the name, like so.
2:45 Now, it won't really join the extension, right,
2:47 it will think that's a separate file,
2:49 so it's up to us to add the extension but other than that,
2:51 it will take an arbitrary number of segments and put them together here
2:54 and let's just print would load form this here,
3:00 ok, so when I run this, and we exit,
3:03 you would see what load I suppose that should be saved to,
3:06 but here we have our ./journals,
3:08 and we can even go one step farther make this very explicit, right,
3:11 if I am going to write that to a log file I need to know
3:14 what the working folder was, at the time, right,
3:17 so I could say path.absolutepath,
3:22 now if I run it, exit out of here,
3:24 you can see user screen caster source python jumpstart... number 4,
3:28 right this is the full path and this works perfectly well in all of the major platforms.
3:35 So this is going to let us work with that folder right there,
3:39 and let's go and say just this, saving to here,
3:44 and put a little indicator that that's not
3:48 proper app output at the terminal there, all right,
3:51 so now we have our file name, we can go and open it,
3:55 and we can go through our journal,
3:57 remember our journal is this list of drinks,
3:59 say for entry in journal data, and say fout.write
4:05 and I can write this out and say entry,
4:07 now, if I say this way it's not quite going to work the way we hope,
4:12 and let's go ahead and leave it for a moment,
4:15 say fout last thing we want to close this
4:18 you always want to close the file handles as soon as you are done with them,
4:21 so let's run this and I'll show you better way to deal with this close scenario.
4:25 So if I run this and let's add an entry,
4:28 say thing 1 and an entry thing 2, and we exit,
4:33 you'll see that this was saved and if we go up here
4:36 there is now this file and if we open it up,
4:38 these are actually all in one line, probably not the best.
4:42 Now, we probably should talk about the format just a bit anyway,
4:44 we want to keep this super simple,
4:47 we are going to get to more interesting file formats later, for our journal,
4:50 we are just going to write one entry per line.
4:53 But notice these are all in the same line
4:55 so we need to actually add a new line character
4:57 so backslash end and then when we rerun this
5:00 say let's add a few entries here, and exit,
5:09 now if we go back to our journal,
5:11 you'll see perfectly misspelled thing 1 and thing 2, excellent.
5:15 Ok, so it looks like our file I/O is working,
5:18 now we want to make sure we call close
5:22 if there is some kind of exception or error or early return
5:25 in this obviously a very small bit of code,
5:28 but even it could have an error, however,
5:31 the more complicated the code gets
5:33 the more helpful this what I am about to show is going to be
5:36 if we return without doing this, well that's kind of a problem, right,
5:39 so what we want to do is in Python
5:42 there is actually this thing called a context manager,
5:45 that will say for this block or suite of code,
5:48 I would like this object to exist,
5:50 and regardless of how you leave it through an exception,
5:53 early return, falling to the bottom,
5:55 I want you to cleanup, and it's up to the item being used
5:59 to determine what cleanup means,
6:01 and you can imagine for files that's like close and flesh the buffer.
6:05 So the way we do that is a little different than this,
6:07 we'll say with, it's kind of reversed,
6:10 we'll say with open file name and same parameters, as fout,
6:15 and then we do a block like so.
6:18 And we won't need this anymore,
6:20 because as soon as we leave this block right here,
6:24 it's defined by the indented section it's going to automatically call close.
6:29 Ok, so let me clean this up a little bit,
6:38 here is our final save, we are going to use os.path.absolutepath
6:42 to get a full path from our relative path we built up with join,
6:46 and in OS we do have these for slashes in here,
6:49 so maybe it would make more sense to write it like so, there we go,
6:53 that way it doesn't depend on for slash being a particular separator,
6:57 so here now we have a proper OS independent style
7:00 we use our context manager, our open method to get the file stream,
7:05 and then we just call write, and we write out the text,
7:08 if we want the binary version, we would say WB for write binary,
7:11 and that's that, we are going to do something very similar for loading it.
7:17 In fact, so much so that we are going to use exactly the same line of code,
7:22 I kind of feel like that should be like a method or something,
7:25 so let's go over here and I could write a method
7:28 in this file or I could go to PyCharm and hit control T
7:32 and say I'd like to create a method let's call it get full path name something like that,
7:39 and you see down here it's returning this,
7:41 and we're calling this file name here, let's just do that same thing up there.
7:45 So, this one is different there,
7:47 usually it will be a file but not always, right, not the first time,
7:50 so we can say something like this,
7:52 if os.path.exists file name then we want to load it,
7:57 otherwise we are going to return this thing.
7:59 Now, we are going to need to initialize that
8:03 and maybe fill the data maybe return the empty version,
8:06 but let's do this here, so now just like before,
8:09 we are going to use our with, but we are going to not use the w,
8:12 we are not trying to write it we are just going to read it,
8:14 and I'm going to call it fin for file input,
8:17 and then we can just say for entry and fin.readlines,
8:23 and that will just read in the lines and let me just print,
8:27 would load and put a little print here entry.
8:30 Now this is not going to work out quite as good as we would like, but let me run it,
8:35 so it would load thing 1 new line, thing 2, new line,
8:39 it turns out that in Python, when you do the read lines
8:43 the new line character is still stuck on the end,
8:46 right that back slash end that we out here,
8:49 or just naturally the file, is on here, it's not removed,
8:53 so we can come over here and say strip,
8:55 maybe we want to be careful and just say rstrip off the end,
8:59 now if I run it, I would say would load thing 1, thing 2,
9:03 those are exactly the entries we were hoping for.
9:06 Ok, so instead of doing this,
9:08 we come down here and say data.apend entry.
9:11 Now if I could just load it up and I say list,
9:14 of yeah, I forgot the r strip, so let's say r strip
9:19 to remove the white space off the right hand side,
9:21 now have a look, perfect, still misspelled,
9:24 thing 2, thing 1 exactly the right order because remember, we are reversing it.
9:28 So let's go and actually add a few more,
9:30 we'll say add this is fun with files, and add one more, today I saved this.
9:39 If we exit out you'll see saving to this location
9:42 and if we would rerun it, and just do a list, boom,
9:45 there are things we wrote, back in our file,
9:48 you can see right, there is the chronological order
9:52 we created which is of course reversed of what we are showing.
9:55 Beautiful, let's go back and just review what we did for load,
9:57 it's even easier than save, wasn't it, ok,
9:59 over here in journal move our to do,
10:03 so we create an empty journal bit of data and then we get the file name
10:07 using our full path function right, ospath. absolute path and join
10:11 and then, we ask if it does exists let's load it,
10:14 using our super simple format of one entry per line,
10:18 and then we are going to return either the populated or empty data set
10:21 and on the way out the door
10:23 we create a writable file and we just write out the format that we were expecting.
10:27 Perfect.